1. Field of the Invention
The present invention concerns a control and method of controlling an air conditioning unit. More specifically, the present invention concerns sensing return air temperature and controlling the air conditioning unit upon startup based upon that temperature.
2. Prior Art
A typical air conditioning unit includes a fan for circulating air to an enclosure to be conditioned and a refrigeration circuit including a compressor, condenser, evaporator and expansion device for absorbing heat energy from the air. Additionally, heating means in the form of electric resistance heaters, fuel fired burners or a reversible refrigeration system may be incorporated into the air conditioning unit. Other features such as economizer operation for drawing cooler ambient air into the enclosure may also be provided. The operation of the unit is typically controlled by a temperature sensing element located either in the enclosure or in the discharge air stream from the air conditioning unit. Based upon the temperatures sensed, the unit is operated in the heating or cooling mode such that the enclosure temperature is maintained in the appropriate range.
In a typical commercial application, the air conditioning unit is operated on a weekly schedule such as five days a week in an office building or seven days a week in a store or other application where there is daily use. To save energy, air conditioning is typically operated only during the hours of use and any time period prior to use for conditioning the air to the appropriate temperature range. In a typical store application an air conditioning unit may be energized at 7:00 A.M. to provide the appropriate enclosure conditions by 9:00 A.M. when the store is opened to the public. The air conditioning unit may then be de-energized at 8:00 P.M. with the store allowed to coast until the 9:00 P.M. closing time.
One type of air conditioning system is controlled based on discharge air temperature from the system. Specifically, variable air volume systems are those systems wherein the supply air is ducted to numerous outlets, each of which modulates to vary the volume of air being discharged therefrom in response to the temperature condition of that area. These devices typically include a bladder type device which opens or closes a port allowing a predetermined volume of air to enter that portion of the enclosure. A single temperature sensing element in the enclosure would be ineffective to regulate the air conditioning unit since the total demand is a function of all the individual variable air volume outlets. Hence, in this type application the temperature being sensed is the discharge temperature from the air conditioning unit which is the temperature of the air being supplied to a supply plenum for conducting the air to the various variable air volume outlets.
It has been found that upon the recurrent energization of the air conditioning unit on a daily basis typically done on a seven day time clock, there is very little cooling load when the unit is first initiated. In fact, upon initiation there may be a brief heating need to bring the enclosure up to the desired temperature.
It has additionally been found that upon initiation of the unit in the morning or startup that the temperature sensed in the supply plenum is typically warm although not necessarily indicative of the enclosure requiring cooling. Since this supply plenum temperature is warm the air conditioning unit is operated in the cooling mode until the supply plenum is reduced in temperature. This operation of the unit may be unnecessary and effect cooling of the enclosure below a desired temperature. In use with a variable air volume system this operation may create additional potential problems since the variable air volume outlets will always discharge a minimum amount of conditioned air flow and hence a cool building may be further cooled at startup based on a discharge plenum thermostat.
Additionally, an occasional heating requirement in a commercial building may be found upon startup. Although the unit senses a heating load it is desirable to have the unit operate only in a heating mode for a brief period to raise the temperature of the enclosure and thereafter to switch to a cooling mode for the remainder of the time interval for which it will operate. In the past, with a variable air volume system it has been possible to operate the unit to supply heat energy which raises the discharge plenum temperature to the point where the unit is then operated in the cooling mode. Hence, the air conditioning unit under these circumstances is operated sequentially in heating and cooling wasting energy and providing little net effect to the enclosure.
The present control scheme provides a system wherein upon startup of the unit on a daily or other recurrent basis a fan is first operated. The fan circulates air for a predetermined time interval, said air flowing from the unit through a supply plenum to the enclosure and through a return plenum back to the unit. A temperature sensor is mounted in the return plenum to determine the temperature of the return air.
If the temperature of the return air is sufficiently high the unit will enter the cooling mode and then be operated from the discharge temperature sensor. If the temperature of the return air is insufficient to require cooling then the discharge temperature sensor will be bypassed preventing the unit from operating in the cooling mode until the return air temperature rises to a predetermined temperature level.
If heating is required, the return air sensor will initiate the heating cycle. Once this cycle is complete the thermostat will allow the unit to be operated in the heating mode to satisfy the heating load. Should the return air sensor detect a temperature sufficiently high enough to indicate that cooling is required the heating mode of operation of the air conditioning unit will be locked out for the remainder of that time interval. Hence, the method and apparatus described herein act to prevent needless, redundant or energy wasteful operation of the air conditioning unit during startup at the beginning of each time interval.